Re: [PATCH] memcg: update documentation v5

[KAMEZAWA Hiroyuki]

On Tue, 13 Apr 2010 11:34:06 +0530
Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-04-13 13:45:53]:
> 
> > Thank you very much for your kindly helps!.
> > -Kame
> > =
> > Documentation update.
> > 
> > Some information are old, and  I think current document doesn't work
> > as "a guide for users".
> > We need summary of all of our controls, at least.
> > 
> > Changelog: 2010/04/12
> > * applied feedback
> > 
> > Changelog: 2010/04/09
> > * replace 'lru' with 'LRU' and 'oom' with 'OOM'
> > * fixed double-space breakage
> > * applied all comments and fixed wrong parts pointed out.
> > * fixed cgroup.procs
> > 
> > Changelog: 2009/04/07
> > * fixed tons of typos.
> > * replaced "memcg" with "memory cgroup" AMAP.
> > * replaced "mem+swap" with "memory+swap"
> > 
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > ---
> >  Documentation/cgroups/memory.txt |  276 ++++++++++++++++++++++++++-------------
> >  1 file changed, 188 insertions(+), 88 deletions(-)
> > 
> > Index: mmotm-temp/Documentation/cgroups/memory.txt
> > ===================================================================
> > --- mmotm-temp.orig/Documentation/cgroups/memory.txt
> > +++ mmotm-temp/Documentation/cgroups/memory.txt
> > @@ -4,16 +4,6 @@ NOTE: The Memory Resource Controller has
> >  to as the memory controller in this document. Do not confuse memory controller
> >  used here with the memory controller that is used in hardware.
> > 
> > -Salient features
> > -
> > -a. Enable control of Anonymous, Page Cache (mapped and unmapped) and
> > -   Swap Cache memory pages.
> > -b. The infrastructure allows easy addition of other types of memory to control
> > -c. Provides *zero overhead* for non memory controller users
> > -d. Provides a double LRU: global memory pressure causes reclaim from the
> > -   global LRU; a cgroup on hitting a limit, reclaims from the per
> > -   cgroup LRU
> > -
> >  Benefits and Purpose of the memory controller
> > 
> >  The memory controller isolates the memory behaviour of a group of tasks
> > @@ -33,6 +23,45 @@ d. A CD/DVD burner could control the amo
> >  e. There are several other use cases, find one or use the controller just
> >     for fun (to learn and hack on the VM subsystem).
> > 
> > +Current Status: linux-2.6.34-mmotm(development version of 2010/April)
> > +
> > +Features:
> > + - accounting anonymous pages, file caches, swap caches usage and limit them.
> > + - private LRU and reclaim routine. (system's global LRU and private LRU
> > +   work independently from each other)
> > + - optionally, memory+swap usage can be accounted and limited.
> > + - hierarchical accounting
> > + - soft limit
> > + - moving(recharging) account at moving a task is selectable.
> > + - usage threshold notifier
> > + - oom-killer disable knob and oom-notifier
> > + - Root cgroup has no limit controls.
> > +
> > + Kernel memory and Hugepages are not under control yet. We just manage
> > + pages on LRU. To add more controls, we have to take care of performance.
> > +
> > +Brief summary of control files.
> > +
> > + tasks				 # attach a task(thread) and show list of threads
> > + cgroup.procs			 # show list of processes
> > + cgroup.event_control		 # an interface for event_fd()
> > + memory.usage_in_bytes		 # show current memory(RSS+Cache) usage.
> > + memory.memsw.usage_in_bytes	 # show current memory+Swap usage
> > + memory.limit_in_bytes		 # set/show limit of memory usage
> > + memory.memsw.limit_in_bytes	 # set/show limit of memory+Swap usage
> > + memory.failcnt			 # show the number of memory usage hits limits
> > + memory.memsw.failcnt		 # show the number of memory+Swap hits limits
> > + memory.max_usage_in_bytes	 # show max memory usage recorded
> > + memory.memsw.usage_in_bytes	 # show max memory+Swap usage recorded
> > + memory.soft_limit_in_bytes	 # set/show soft limit of memory usage
> > + memory.stat			 # show various statistics
> > + memory.use_hierarchy		 # set/show hierarchical account enabled
> > + memory.force_empty		 # trigger forced move charge to parent
> > + memory.swappiness		 # set/show swappiness parameter of vmscan
> > + 				 (See sysctl's vm.swappiness)
> > + memory.move_charge_at_immigrate # set/show controls of moving charges
> > + memory.oom_control		 # set/show oom controls.
> > +
> 
> Can we align the "#" comments, please!
> 
It's aligned.


> >  1. History
> > 
> >  The memory controller has a long history. A request for comments for the memory
> > @@ -106,14 +135,14 @@ the necessary data structures and check 
> >  is over its limit. If it is then reclaim is invoked on the cgroup.
> >  More details can be found in the reclaim section of this document.
> >  If everything goes well, a page meta-data-structure called page_cgroup is
> > -allocated and associated with the page.  This routine also adds the page to
> > -the per cgroup LRU.
> > +updated. page_cgroup has its own LRU on cgroup.
> > +(*) page_cgroup structure is allocated at boot/memory-hotplug time.
> > 
> >  2.2.1 Accounting details
> > 
> >  All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
> > -(some pages which never be reclaimable and will not be on global LRU
> > - are not accounted. we just accounts pages under usual vm management.)
> > +Some pages which are never reclaimable and will not be on the global LRU
> > +are not accounted. We just account pages under usual VM management.
> > 
> >  RSS pages are accounted at page_fault unless they've already been accounted
> >  for earlier. A file page will be accounted for as Page Cache when it's
> > @@ -121,12 +150,19 @@ inserted into inode (radix-tree). While 
> >  processes, duplicate accounting is carefully avoided.
> > 
> >  A RSS page is unaccounted when it's fully unmapped. A PageCache page is
> > -unaccounted when it's removed from radix-tree.
> > +unaccounted when it's removed from radix-tree. Even if RSS pages are fully
> > +unmapped (by kswapd), they may exist as SwapCache in the system until they
> > +are really freed. Such SwapCaches also also accounted.
> > +A swapped-in page is not accounted until it's mapped.
> > +
> > +Note: The kernel does swapin-readahead and read multiple swaps at once.
> > +This means swapped-in pages may contain pages for other tasks than a task
> > +causing page fault. So, we avoid accounting at swap-in I/O.
> > 
> >  At page migration, accounting information is kept.
> > 
> > -Note: we just account pages-on-lru because our purpose is to control amount
> > -of used pages. not-on-lru pages are tend to be out-of-control from vm view.
> > +Note: we just account pages-on-LRU because our purpose is to control amount
> > +of used pages. not-on-LRU pages tend to be out-of-control from VM view.
> 
>                 ^^ period might not be appropriate, if it is, n (not) should
> be caps

replaced with ,

> > 
> >  2.3 Shared Page Accounting
> > 
> > @@ -143,6 +179,7 @@ caller of swapoff rather than the users 
> > 
> > 
> >  2.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP)
> > +
> >  Swap Extension allows you to record charge for swap. A swapped-in page is
> >  charged back to original page allocator if possible.
> > 
> > @@ -150,13 +187,20 @@ When swap is accounted, following files 
> >   - memory.memsw.usage_in_bytes.
> >   - memory.memsw.limit_in_bytes.
> > 
> > -usage of mem+swap is limited by memsw.limit_in_bytes.
> > +memsw means memory+swap. Usage of memory+swap is limited by
> > +memsw.limit_in_bytes.
> > 
> > -* why 'mem+swap' rather than swap.
> > +Example: Assume a system with 4G of swap. A task which allocates 6G of memory
> > +(by mistake) under 2G memory limitation will use all swap.
> > +In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
> > +By using memsw limit, you can avoid system OOM which can be caused by swap
> > +shortage.
> > +
> > +* why 'memory+swap' rather than swap.
> >  The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
> >  to move account from memory to swap...there is no change in usage of
> > -mem+swap. In other words, when we want to limit the usage of swap without
> > -affecting global LRU, mem+swap limit is better than just limiting swap from
> > +memory+swap. In other words, when we want to limit the usage of swap without
> > +affecting global LRU, memory+swap limit is better than just limiting swap from
> >  OS point of view.
> > 
> >  * What happens when a cgroup hits memory.memsw.limit_in_bytes
> > @@ -168,12 +212,12 @@ it by cgroup.
> > 
> >  2.5 Reclaim
> > 
> > -Each cgroup maintains a per cgroup LRU that consists of an active
> > -and inactive list. When a cgroup goes over its limit, we first try
> > +Each cgroup maintains a per cgroup LRU which has the same structure as
> > +global VM. When a cgroup goes over its limit, we first try
> >  to reclaim memory from the cgroup so as to make space for the new
> >  pages that the cgroup has touched. If the reclaim is unsuccessful,
> >  an OOM routine is invoked to select and kill the bulkiest task in the
> > -cgroup.
> > +cgroup. (See 10. OOM Control below.)
> > 
> >  The reclaim algorithm has not been modified for cgroups, except that
> >  pages that are selected for reclaiming come from the per cgroup LRU
> > @@ -187,13 +231,19 @@ Note2: When panic_on_oom is set to "2", 
> >  When oom event notifier is registered, event will be delivered.
> >  (See oom_control section)
> > 
> > -2. Locking
> > +2.6 Locking
> > 
> > -The memory controller uses the following hierarchy
> > +   lock_page_cgroup()/unlock_page_cgroup() should not be called under
> > +   mapping->tree_lock.
> > 
> > -1. zone->lru_lock is used for selecting pages to be isolated
> > -2. mem->per_zone->lru_lock protects the per cgroup LRU (per zone)
> > -3. lock_page_cgroup() is used to protect page->page_cgroup
> > +   Other lock order is following:
> > +   PG_locked.
> > +   mm->page_table_lock
> > +       zone->lru_lock
> > +	  lock_page_cgroup.
> > +  In many cases, just lock_page_cgroup() is called.
> > +  per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by
> > +  zone->lru_lock, it has no lock of its own.
> > 
> >  3. User Interface
> > 
> > @@ -202,6 +252,7 @@ The memory controller uses the following
> >  a. Enable CONFIG_CGROUPS
> >  b. Enable CONFIG_RESOURCE_COUNTERS
> >  c. Enable CONFIG_CGROUP_MEM_RES_CTLR
> > +d. Enable CONFIG_CGROUP_MEM_RES_CTLR_SWAP (to use swap extension)
> > 
> >  1. Prepare the cgroups
> >  # mkdir -p /cgroups
> > @@ -209,31 +260,29 @@ c. Enable CONFIG_CGROUP_MEM_RES_CTLR
> > 
> >  2. Make the new group and move bash into it
> >  # mkdir /cgroups/0
> > -# echo $$ >  /cgroups/0/tasks
> > +# echo $$ > /cgroups/0/tasks
> > 
> >  Since now we're in the 0 cgroup,
> >  We can alter the memory limit:
> >  # echo 4M > /cgroups/0/memory.limit_in_bytes
> > 
> >  NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
> > -mega or gigabytes.
> > +mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)
> > +
> >  NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
> >  NOTE: We cannot set limits on the root cgroup any more.
> > 
> >  # cat /cgroups/0/memory.limit_in_bytes
> >  4194304
> > 
> > -NOTE: The interface has now changed to display the usage in bytes
> > -instead of pages
> > -
> >  We can check the usage:
> >  # cat /cgroups/0/memory.usage_in_bytes
> >  1216512
> > 
> >  A successful write to this file does not guarantee a successful set of
> > -this limit to the value written into the file.  This can be due to a
> > +this limit to the value written into the file. This can be due to a
> >  number of factors, such as rounding up to page boundaries or the total
> > -availability of memory on the system.  The user is required to re-read
> > +availability of memory on the system. The user is required to re-read
> >  this file after a write to guarantee the value committed by the kernel.
> > 
> >  # echo 1 > memory.limit_in_bytes
> > @@ -248,15 +297,25 @@ caches, RSS and Active pages/Inactive pa
> > 
> >  4. Testing
> > 
> > -Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11].
> > -Apart from that v6 has been tested with several applications and regular
> > -daily use. The controller has also been tested on the PPC64, x86_64 and
> > -UML platforms.
> > +For testing features and implementation, see memcg_test.txt.
> > +
> > +Performance test is also important. To see pure memory cgroup's overhead,
> > +testing on tmpfs will give you good numbers of small overheads.
> > +Example: do kernel make on tmpfs.
> > +
> > +Page-fault scalability is also important. At measuring parallel
> > +page fault test, multi-process test may be better than multi-thread
> > +test because it has noise of shared objects/status.
> > +
> > +But the above two are testing extreme situations.
> > +Trying usual test under memory cgroup is always helpful.
> > +
> > +
> 
> Extra newline.
> 
will remove

> > 
> >  4.1 Troubleshooting
> > 
> >  Sometimes a user might find that the application under a cgroup is
> > -terminated. There are several causes for this:
> > +terminated by OOM killer. There are several causes for this:
> > 
> >  1. The cgroup limit is too low (just too low to do anything useful)
> >  2. The user is using anonymous memory and swap is turned off or too low
> > @@ -264,6 +323,9 @@ terminated. There are several causes for
> >  A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
> >  some of the pages cached in the cgroup (page cache pages).
> > 
> > +To know what happens, disable OOM_Kill by 10. OOM Control(see below) and
> > +seeing what happens will be helpful.
> > +
> >  4.2 Task migration
> > 
> >  When a task migrates from one cgroup to another, it's charge is not
> > @@ -271,16 +333,19 @@ carried forward by default. The pages al
> >  remain charged to it, the charge is dropped when the page is freed or
> >  reclaimed.
> > 
> > -Note: You can move charges of a task along with task migration. See 8.
> > +You can move charges of a task along with task migration.
> > +See 8. "Move charges at task migration"
> > 
> >  4.3 Removing a cgroup
> > 
> >  A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
> >  cgroup might have some charge associated with it, even though all
> > -tasks have migrated away from it.
> > -Such charges are freed(at default) or moved to its parent. When moved,
> > -both of RSS and CACHES are moved to parent.
> > -If both of them are busy, rmdir() returns -EBUSY. See 5.1 Also.
> > +tasks have migrated away from it. (because we charge against pages, not
> > +against tasks.)
> > +
> > +Such charges are freed or moved to their parent. At moving, both of RSS
> > +and CACHES are moved to parent.
> > +rmdir() may return -EBUSY if freeing/moving fails. See 5.1 also.
> > 
> >  Charges recorded in swap information is not updated at removal of cgroup.
> >  Recorded information is discarded and a cgroup which uses swap (swapcache)
> > @@ -296,10 +361,10 @@ will be charged as a new owner of it.
> > 
> >    # echo 0 > memory.force_empty
> > 
> > -  Almost all pages tracked by this memcg will be unmapped and freed. Some of
> > -  pages cannot be freed because it's locked or in-use. Such pages are moved
> > -  to parent and this cgroup will be empty. But this may return -EBUSY in
> > -  some too busy case.
> > +  Almost all pages tracked by this memory cgroup will be unmapped and freed.
> > +  Some pages cannot be freed because they are locked or in-use. Such pages are
> > +  moved to parent and this cgroup will be empty. This may return -EBUSY if
> > +  VM is too busy to free/move all pages immediately.
> > 
> >    Typical use case of this interface is that calling this before rmdir().
> >    Because rmdir() moves all pages to parent, some out-of-use page caches can be
> > @@ -309,19 +374,41 @@ will be charged as a new owner of it.
> > 
> >  memory.stat file includes following statistics
> > 
> > +# per-memory cgroup local status
> >  cache		- # of bytes of page cache memory.
> >  rss		- # of bytes of anonymous and swap cache memory.
> > +mapped_file	- # of bytes of mapped file (includes tmpfs/shmem)
> >  pgpgin		- # of pages paged in (equivalent to # of charging events).
> >  pgpgout		- # of pages paged out (equivalent to # of uncharging events).
> > -active_anon	- # of bytes of anonymous and  swap cache memory on active
> > -		  lru list.
> > +swap		- # of bytes of swap usage
> >  inactive_anon	- # of bytes of anonymous memory and swap cache memory on
> > -		  inactive lru list.
> > -active_file	- # of bytes of file-backed memory on active lru list.
> > -inactive_file	- # of bytes of file-backed memory on inactive lru list.
> > +		LRU list.
> > +active_anon	- # of bytes of anonymous and swap cache memory on active
> > +		inactive LRU list.
> > +inactive_file	- # of bytes of file-backed memory on inactive LRU list.
> > +active_file	- # of bytes of file-backed memory on active LRU list.
> >  unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).
> > 
> > -The following additional stats are dependent on CONFIG_DEBUG_VM.
> > +# status considering hierarchy (see memory.use_hierarchy settings)
> > +
> > +hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
> > +			under which the memory cgroup is
> > +hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
> > +			hierarchy under which memory cgroup is.
> > +
> > +total_cache		- sum of all children's "cache"
> > +total_rss		- sum of all children's "rss"
> > +total_mapped_file	- sum of all children's "cache"
> > +total_pgpgin		- sum of all children's "pgpgin"
> > +total_pgpgout		- sum of all children's "pgpgout"
> > +total_swap		- sum of all children's "swap"
> > +total_inactive_anon	- sum of all children's "inactive_anon"
> > +total_active_anon	- sum of all children's "active_anon"
> > +total_inactive_file	- sum of all children's "inactive_file"
> > +total_active_file	- sum of all children's "active_file"
> > +total_unevictable	- sum of all children's "unevictable"
> > +
> > +# The following additional stats are dependent on CONFIG_DEBUG_VM.
> > 
> >  inactive_ratio		- VM internal parameter. (see mm/page_alloc.c)
> >  recent_rotated_anon	- VM internal parameter. (see mm/vmscan.c)
> > @@ -330,24 +417,37 @@ recent_scanned_anon	- VM internal parame
> >  recent_scanned_file	- VM internal parameter. (see mm/vmscan.c)
> >
> 
> Can we align the data on the right, like for total_* data earlier.
>  
It's aligned.


> >  Memo:
> > -	recent_rotated means recent frequency of lru rotation.
> > -	recent_scanned means recent # of scans to lru.
> > +	recent_rotated means recent frequency of LRU rotation.
> > +	recent_scanned means recent # of scans to LRU.
> >  	showing for better debug please see the code for meanings.
> > 
> >  Note:
> >  	Only anonymous and swap cache memory is listed as part of 'rss' stat.
> >  	This should not be confused with the true 'resident set size' or the
> > -	amount of physical memory used by the cgroup. Per-cgroup rss
> > -	accounting is not done yet.
> > +	amount of physical memory used by the cgroup.
> > +	'rss + file_mapped" will give you resident set size of cgroup.
> > +	(Note: file and shmem may be shared among other cgroups. In that case,
> > +	 file_mapped is accounted only when the memory cgroup is owner of page
> > +	 cache.)
> > 
> >  5.3 swappiness
> > -  Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
> > 
> > -  Following cgroups' swappiness can't be changed.
> > -  - root cgroup (uses /proc/sys/vm/swappiness).
> > -  - a cgroup which uses hierarchy and it has child cgroup.
> > -  - a cgroup which uses hierarchy and not the root of hierarchy.
> > +Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
> > +
> > +Following cgroups' swappiness can't be changed.
> > +- root cgroup (uses /proc/sys/vm/swappiness).
> > +- a cgroup which uses hierarchy and it has other cgroup(s) below it.
> > +- a cgroup which uses hierarchy and not the root of hierarchy.
> > +
> > +5.4 failcnt
> > +
> > +A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
> > +This failcnt(== failure count) shows the number of times that a usage counter
> > +hit its limit. When a memory cgroup hits a limit, failcnt increases and
> > +memory under it will be reclaimed.
> > 
> > +You can reset failcnt by writing 0 to failcnt file.
> > +# echo 0 > .../memory.failcnt
> > 
> >  6. Hierarchy support
> > 
> > @@ -366,13 +466,13 @@ hierarchy
> > 
> >  In the diagram above, with hierarchical accounting enabled, all memory
> >  usage of e, is accounted to its ancestors up until the root (i.e, c and root),
> > -that has memory.use_hierarchy enabled.  If one of the ancestors goes over its
> > +that has memory.use_hierarchy enabled. If one of the ancestors goes over its
> >  limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
> >  children of the ancestor.
> > 
> >  6.1 Enabling hierarchical accounting and reclaim
> > 
> > -The memory controller by default disables the hierarchy feature. Support
> > +A memory cgroup by default disables the hierarchy feature. Support
> >  can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
> > 
> >  # echo 1 > memory.use_hierarchy
> > @@ -382,10 +482,10 @@ The feature can be disabled by
> >  # echo 0 > memory.use_hierarchy
> > 
> >  NOTE1: Enabling/disabling will fail if the cgroup already has other
> > -cgroups created below it.
> > +       cgroups created below it.
> > 
> >  NOTE2: When panic_on_oom is set to "2", the whole system will panic in
> > -case of an oom event in any cgroup.
> > +       case of an OOM event in any cgroup.
> > 
> >  7. Soft limits
> > 
> > @@ -395,7 +495,7 @@ is to allow control groups to use as muc
> >  a. There is no memory contention
> >  b. They do not exceed their hard limit
> > 
> > -When the system detects memory contention or low memory control groups
> > +When the system detects memory contention or low memory, control groups
> >  are pushed back to their soft limits. If the soft limit of each control
> >  group is very high, they are pushed back as much as possible to make
> >  sure that one control group does not starve the others of memory.
> > @@ -409,7 +509,7 @@ it gets invoked from balance_pgdat (kswa
> >  7.1 Interface
> > 
> >  Soft limits can be setup by using the following commands (in this example we
> > -assume a soft limit of 256 megabytes)
> > +assume a soft limit of 256 MiB)
> > 
> >  # echo 256M > memory.soft_limit_in_bytes
> > 
> > @@ -418,7 +518,7 @@ If we want to change this to 1G, we can 
> >  # echo 1G > memory.soft_limit_in_bytes
> > 
> >  NOTE1: Soft limits take effect over a long period of time, since they involve
> > -       reclaiming memory for balancing between memory cgroups
> > +reclaiming memory for balancing between memory cgroups
> >  NOTE2: It is recommended to set the soft limit always below the hard limit,
> >         otherwise the hard limit will take precedence.
> > 
> > @@ -445,7 +545,7 @@ Note: Charges are moved only when you mo
> >  Note: If we cannot find enough space for the task in the destination cgroup, we
> >        try to make space by reclaiming memory. Task migration may fail if we
> >        cannot make enough space.
> > -Note: It can take several seconds if you move charges in giga bytes order.
> > +Note: It can take several seconds if you move charges much.
> > 
> >  And if you want disable it again:
> > 
> > @@ -476,15 +576,15 @@ Note: More type of pages(e.g. file cache
> > 
> >  9. Memory thresholds
> > 
> > -Memory controler implements memory thresholds using cgroups notification
> > +Memory cgroup implements memory thresholds using cgroups notification
> >  API (see cgroups.txt). It allows to register multiple memory and memsw
> >  thresholds and gets notifications when it crosses.
> > 
> >  To register a threshold application need:
> > - - create an eventfd using eventfd(2);
> > - - open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
> > - - write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to
> > -   cgroup.event_control.
> > +- create an eventfd using eventfd(2);
> > +- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
> > +- write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to
> 
> Do we need the <> around memory.usage_in_bytes
> 

Hmm ? I'm not sure. 

Could you explain why you think removing <> is better ?

Thanks,
-Kame

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